During the production of metal compounds it is known to use thermal processes to remove volatile metal compounds from feed materials. For example, thermal processes can involve the use of chemical reactions to convert metal oxides into their corresponding elemental metals. When the elemental metals are volatile at the operating temperatures of the process, these volatile metals can be removed along with other compounds being processed. For example, metal halides that may be present have boiling points that are typically lower than the metal oxides in the feed materials and thus can be removed together as a vapor stream together with other volatile metals. Thermal processes thus can be used to remove specific metals present in process feed materials as volatile metals and metal compounds.
As the volatile metals are removed from the less volatile components of the feed materials, they can be reacted with air to form metal oxides which can be subsequently processed to recover materials of interest. This type of thermal processing method has been used in the refinement of some ores and in the processing of waste materials to remove specific materials of interest. The volatile metals and metal compounds that are thermally released in such processes are normally allowed to cool and condense. Once condensed, the metal compounds can be collected in a dust collection system for use or disposal.
Electric arc furnace (EAF) dust is a waste material that is generated during the production of steel in electric arc furnaces. During the production of steel, EAF dust is captured and collected for either disposal or recycle.
The major components in EAF dust are iron (up to 50 wt. %) and zinc (up to 30 wt. %), usually in the oxide form. In addition, EAF dust contains smaller quantities of calcium, magnesium, manganese, lead, cadmium, chloride, fluoride and other trace elements.
Generally, it can be economical to process EAF dust having a high zinc content using existing zinc recovery procedures, and send EAF dust having a low zinc content to be chemically stabilized and landfilled for disposal as a waste.
The thermal processes discussed above have been used in the processing of EAF dust to remove volatile metals including zinc, lead and cadmium. The EAF dust is processed by adding a reducing material thereto and then heating the resulting mixture. As desirable volatile metals are formed, they are released from the mixture, along with other less desirable and/or undesirable volatile materials, such as metal halides. As the volatile materials leave the process, they are collected and mixed with air or oxygen so that at least the elemental volatile metal components form metal oxides in a hot gaseous stream. The gaseous stream is then cooled to condense the mixture of compounds and the condensed material is collected in a dust collection system.
The material collected from processing of EAF dust as discussed above is referred to as a crude zinc oxide material and typically comprises zinc oxide, zinc chloride, lead chloride, cadmium and cadmium chloride. While this mixture has some commercial value depending on the composition, the value of a high purity zinc oxide would be much higher and is desired.
The separation of the crude zinc oxide material into more valuable high purity zinc oxide can be performed in a second separate step using either a second thermal or wet chemical process. However, the use of a second process increases costs proportionally to the additional energy that must be spent to separate the compounds from the mixture. Thermal processing of crude zinc oxide materials would involve the heating of the material in an oxidizing atmosphere to a temperature in excess of the boiling temperature of the impurities to be removed. In the case of crude zinc oxide material, the material would have to be heated to a temperature of over 1800° F. in order to volatilize lead halides, cadmium halides, cadmium and zinc chloride. This heating process could be performed in a rotary kiln, tunnel kiln, rotary hearth furnace or similar heating apparatus.
An alternative to thermal processing would be to separate specific metal oxides from impurities using a wet chemical process to dissolve and wash impurities away from the desired metal oxide products. This washing would be performed at specific pH levels to leave the desired metal in the oxide form while washing away impurities such as halides.
The following patents exemplify various systems that process volatile metals: U.S. Pat. No. 4,108,634 to Rahn et al., U.S. Pat. No. 4,200,454 to Maelzer et al., U.S. Pat. No. 5,474,592 to Bresser et al., U.S. Pat. No. 5,567,225 to Bernard et al., U.S. Pat. No. 5,667,553 to Keegel, Jr., U.S. Pat. No. 5,766,307 to Miyagawa et al., U.S. Pat. No. 6,395,060 to Home, et al., and U.S. Pat. No. 6,464,753 to Home et al. In addition, pending patent application Ser. No. 2002/001133 to Horn et al. discloses a system for processing volatile metals.
The present invention provides processes and apparatus for the separation of volatile metals, from mixtures thereof which involve the use of mechanical or physical separation devices and systems.